1. Field of the Invention
The present invention relates to an optical scanning device, a method for adjusting an error, and an image forming apparatus.
2. Description of the Related Art
Recently, there has been a demand to increase the printing speed and improve the image quality (image density) in image forming apparatuses such as laser printers and digital copiers. To cope with this demand, a new technique was developed that employs a multibeam light source, instead of a single light source. The multibeam light source emits a plurality of light beams simultaneously. To back up this new technique, various optical systems compatible with multi beams light sources have been developed.
For example, Japanese Patent No. 3445050 discloses a multibeam scanning optical device that reduces changes in F number in a sub-scanning direction in accordance with an image height produced by an incident beam onto a scan target. This has been achieved by continuously changing a curvature in a sub scanning direction from on-axis toward off-axis for both lens surfaces of a single lens.
Japanese Patent No. 3768734 discloses an optical scanning device that employs a scanning image-forming lens having a doublet lens or a multiplet lens that has at least two special surfaces on which the sub-scanning curvature changes from the optical axis of the lens toward the periphery of the lens in the main scanning direction. On at least one of the special surfaces, the change in the sub-scanning curvature is asymmetrical relative to the main scanning direction. Moreover, the sub-scanning curvature has a plurality of extreme values.
Japanese Patent Application Laid-open No. 2005-338865 discloses an optical scanning optical device that reduces changes in F number in the sub-scanning direction in accordance with an image height produced by an incident beam onto a scan target. This has been achieved by making the position of an on-axis main plane in the optical axis direction in the sub-scanning direction closer to the scan target than the position of an off-axis main plane in the sub-scanning direction, and by continuously changing a curvature in the sub scanning direction from on-axis toward off-axis.
Finally, Japanese Patent Application Laid-open No. 2002-287055 discloses an optical scanning device that includes a light source on which a plurality of light emitting elements is arranged two-dimensionally to scan the same scanning line or different scanning lines and a defecting unit that collectively scans a plurality of light beams from the light source in a main direction. The optical scanning device rotates and adjusts the light source around an optical axis, whereby an exposure energy distribution in a sub-scanning direction formed by the light beams in the same scanning substantially equals to a desirable exposure energy distribution.
A light source that has a plurality of the light emitting elements arranged one-dimensionally can adjust all the intervals between the light emitting elements in a sub-scanning direction simultaneously by rotating the light source around an optical axis direction as a rotation axis (i.e., γ-rotation). By contrast, a light source that has a plurality of light emitting elements arranged two-dimensionally in a high density such as so-called a surface-emitting laser array is advantageous for high density writing. However, the two-dimensionally arranged light emitting elements have difficulties in adjusting intervals of scanning lines on a scan target to be uniform with high precision that is to be required for obtaining high quality images by the γ-rotation because intervals between the light emitting elements do not change uniformly when γ-rotation is performed (see FIG. 12). Intervals between the light emitting elements means a center-to-center distance between two adjacent light emitting elements.